The cave dwellers' diet was found to include yarrow and chamomile, both bitter-tasting plants with little nutritional value. Earlier research by the same team had shown that the Neanderthals in El Sidrón had a gene for tasting bitter substances.

"We know that Neanderthals would find these plants bitter, so it is likely these plants must have been selected for reasons other than taste"—probably medication, Hardy said in a statement.

"It fits in well with the behavioral pattern of self-medication by today's higher primates, and indeed many other animals."

It's impossible to know what cures Neanderthals sought from the plants, but people use them today to treat a variety of ailments, she noted.

"Chamomile is very well known as a herbal treatment for nerves and stress, and for digestive disorders," while yarrow is used to treat colds and fevers and works as an antiseptic, she said.

Veggie-Loving Neanderthals

The research adds to recent findings that question the Neanderthals' reputation as inflexible carnivores—previously cited as a reason why modern humans, able to draw on a wider variety of food sources, gained a competitive edge over their heavy-browed cousins.

"Our results do add to the growing picture of plant consumption by Neanderthals," said Hardy, who worked with archaeological chemist Stephen Buckley of the University of York.